1. Field of the Invention
The present invention relates to an engine speed control device and an engine speed control method; the present invention especially relates to the engine speed control device and the engine speed control method of a small size engine that is used for general purpose and is used as a power source of a machine such as a generator, a pump or a mower.
2. Background of the Invention
As a small size engine that is used for general purpose and is used as a power source of a machine such as a generator, a pump or a mower, an engine provided with an electric governor is conventionally known, the throttle valve of the carburetor in the engine being operated by an actuator. In this specification, the machine such as described just above is to be called a farm working machine.
The small size general-purpose engine provided with the electric governor is operated so that the engine speed is maintained at a constant level by regulating the fuel injection rate (almost equivalent to the engine torque T) corresponding to the engine speed even when the engine load changes. To be more specific, the electric governor controls the opening of the throttle valve by use of a feedback control manner in which the fuel injection rate is increased or decreased so as to eliminate the difference between the established target engine-speed and the actual engine speed.
In the small size general-purpose engine driving such a farm working machine as is described above, when the power demand from the farm working machine is zero while the engine is rotated, namely, when the rotating engine generates no load while the movement of the farm working machine is temporarily suspended, it is desirable to improve the fuel consumption and reduce the engine noises by reducing the engine speed.
Patent Reference 1, for instance, discloses a gasoline engine in which an electric governor regulates the opening of the throttle so that the engine speed is maintained at an idling speed in a case where a switch to detect an idling down (slow-down) order is set at the idling down position.
Further, Patent References 2 as well as 3 discloses a technology regarding idling speed control, although each patent Reference does not assume that the engine disclosed in each patent Reference is connected to a farm working machine as described above.
Patent Reference 2 discloses an idling speed control device of an engine provided with an alternator control device that maintains the field current in the alternator driven by the engine, at a prescribed current level regardless of the fluctuations in the electric load, when a learning-correcting device is activated.
Further, Patent Reference 3 discloses an idling speed control method in which the internal combustion engine performs a feedback control so that the adding-subtracting correction term is established at an appropriate level in response to the loading condition in real time.
In the technology of Patent Reference 3, whether the engine is operated in a loading condition or in no load condition is judged, for instance, based on whether the selector position regarding the automatic transmission of a fluid coupling is placed in a neutral range or in a drive range.